In the integrated circuit industry today, hundreds of thousands of semiconductor devices are built on a single chip. Every device on the chip must be electrically isolated to ensure that it operates independently without interfering with another. The art of isolating semiconductor devices has become an important aspect of modern metal-oxide-semiconductor (MOS) and bipolar integrated circuit technology for the separation of different devices or different functional regions. With the high integration of the semiconductor devices, improper electrical isolation among devices will cause current leakage, and the current leakage can consume a significant amount of power as well as compromise functionality. Among some examples of reduced functionality include latch-up, which can damage the circuit temporarily or permanently, noise margin degradation, voltage shift and cross-talk.
Shallow trench isolation (STI), is the preferred electrical isolation technique especially for a semiconductor chip with high integration. In general, conventional methods of producing an STI feature include first forming a hard mask over the semiconductor substrate, for example, silicon. A trench etching pattern is photolithographically patterned formed over a hard mask dielectric layer, followed by etching through the dielectric hard mask and thereafter etching a trench in the semiconducting substrate surrounding active regions to form an STI feature. Subsequently, the photoresist etching mask is removed and the STI feature is back-filled with a dielectric insulating material.
Frequently STI features are etched with a sequential process flow, where the hard mask layers are etched in one chamber and the silicon trench is etched in another chamber. Etching is frequently performed by a plasma enhanced etching process, for example reactive ion etching (RIE). Typically, in a plasma etching process an etchant source gas supplied to an etching chamber where the plasma is ignited to generate ions from the etchant source gas. Ions are then accelerated towards the process wafer substrate, frequently by a voltage bias, where they remove material (etch) from the process wafer. Various gas chemistries are used to provide variable etching rates for different etching target materials. Frequently used etchant sources include chlorine and bromine based etchants, for example Cl2 and HBr.
It is frequently desirable to use an etching endpoint detection system in plasma enhanced etching to control a depth of etching. For example, For example, in plasma etching endpoint detection systems monitor parameters such as a change in the etch rate, the type of etch products, or a change in the active reactants in the gas discharge. For example, optical emission spectroscopy (OES) has been widely used for endpoint detection by monitoring the intensity of wavelengths of either reactive species or etching products. More specifically, during an RIE process, plasma discharge materials, such as etchant, neutral, and reactive ions in the plasma, are continuously excited by electrons and collisions, giving off emissions ranging from ultraviolet to infrared radiation. An optical emission spectrometer diffracts this light into its component wavelengths and determines the intensity at a particular wavelength. Since each species emits light at a wavelength characteristic only of that species, it is possible to associate a certain wavelength with a particular species, and to use this information to detect an etching endpoint.
In forming a feature in a layer of material in a multilayer semiconductor device, an etching process typically etches away one type of material included in one layer of the device until another different layer of material, which typically has a low selectivity to etching, is reached, for example, an etch stop layer. Since the etch stop layer is typically a different material having a low reactivity with the reactive ion species, a discernible change in either or both the concentration of reactive ion species and the reactively etched species may be easily observed. Since the concentration of given plasma species is proportional to an emitted wavelength of light, the concentration of a given species may be tracked by monitoring the intensity at a given wavelength.
One problem with end-point detection using OES or any other plasma species monitoring method occurs where no change in material during etching, such as a material interface, is present during the etching process thereby providing no readily discernable change in reactive species or reactively etched (product) species. There are several applications, however, where no material interface is present, yet a particular feature etching depth is required, for example, in etching a STI trench in a silicon substrate.
According to prior art processes for anisotropically etching STI trench features, a predetermined etching time period is pre-programmed with associated plasma operating conditions for achieving a pre-determined etching depth. Control of the depth of STI features is critical to proper integrated circuit device functioning including achieving a proper sidewall taper and rounded corners at the trench bottoms and tops to improve electrical isolation properties. As device sizes are diminished, using pre-determined time based dry etching (plasma enhanced) makes precise trench depths increasingly difficult to control. Small variations in the plasma parameters during the etching process can lead to significant trench depth variations. Slight variations in plasma etching parameters are magnified over an etching time period to result in a significant deviation from the desired trench depth.
There is therefore a need in the integrated circuit semiconductor device processing art to develop a plasma enhanced etching process whereby etching depths can be more precisely controlled.
It is therefore an object of the invention to provide a plasma enhanced etching process whereby etching depths can be more precisely controlled while overcoming other shortcomings and deficiencies in the prior art.